Recruitment of motor units in voluntary muscle contraction takes place in an orderly fashion. This process is dominated by differences in motor unit (alpha motoneuron) size and by spinal mechanisms regulating the motoneuron firing rate as a function of its excitatory state. Motor unit sizes, firing rates and recruitment thresholds have been measured precisely in a number of human muscle groups. This is accomplished by recording individual motor unit action potentials during a voluntary contraction and by extracting the unit contribution from the total muscular activity by means of an averaging technique. Unit size estimates are also derived from gradients in the firing rate behavior of recruited units, as a function of voluntary effort. Differences among motor units, in terms of twitch contraction time, fatigue resistance and the degree of motor unit synchronization, are measured as well. Combined with a functional representation of the muscle contractile response, these data complete an accurate model of the normal voluntary force production process. An automated technique has been developed to extract from the data, by computer, a reliable estimate of the motor unit size composition of several human skeletal muscles and muscle groups. Knowledge of the behavior of a relatively small number of motor units suffices for obtaining a clinically useful measure of the degree of normality of the motor unit composition. It is proposed to: 1) further develop this technique and obtain standards for several muscles or muscle groups, 2) document a number of chronic as well as transient pathological processes leading to changes in either the size distribution and or the normal re(de)cruitment cycle and 3) examine systematic effects modulating unit recruitment under different volitional circumstances.